1. Field of the Invention
The present invention relates to a method for fabricating a prism and a method for fabricating an optical system that employs a prism. More particularly, the present invention relates to a method for fabricating a prism and an optical system through which blue light of wavelengths of 420 nm or shorter (in the 405 nm band) is passed.
2. Description of the Prior Art
In an optical disk apparatus for recording and reproducing signals to and from an optical disk such as a DVD, writing and reading of signals are achieved by the use of light of different wavelengths according to the type of medium. U.S. Pat. No. 6,005,835 and other patent publications disclose constructions for reading signals from different types of optical disks by the use of a single optical pickup. FIG. 1 is a diagram showing the construction of such an optical pickup.
The optical pickup 1 has a first light source 2 that emits light of a first wavelength and a second light source 3 that emits light of a second wavelength. The light of the first wavelength is, for example, light in the 650 nm band (red light). This permits reading of signals from a disk D when the disk D is a DVD-ROM. The light of the second wavelength is, for example, light in the 780 nm band (infrared light). This permits reading of signals from the disk D when the disk D is a CD-ROM.
In the optical paths of the light emitted from the first and second light sources 2 and 3, there is disposed a dichroic mirror 5 that reflects the light of the first wavelength and that transmits the light of the second wavelength. Thus, the light emitted from the first light source 2 is directed to the disk D by being reflected from the dichroic mirror 5, and the light emitted from the second light source 3 is directed to the disk D by being transmitted through the dichroic mirror 5.
Between the dichroic mirror 5 and the disk D, there are disposed a prism 8, a collimator lens 4, a quarter-wavelength plate 10, a diffraction grating 7, and a condenser lens 11. The prism 8 is composed of translucent substrates 8a, 8b, and 8c bonded together, and these translucent substrates 8a, 8b, and 8c have surfaces inclined relative to the optical path.
At the interface between the substrates 8a and 8b, there is provided a PBS (polarizing beam splitter) film 8d that transmits P-polarized light and that reflects S-polarized light. At the interface between the substrates 8b and 8c, there is provided a BS (beam splitter) film 8e that reflects part of the light incident thereon and that transmits the remainder thereof.
The collimator lens 4 shapes the light, a divergent beam, coming from the first and second light sources 2 and 3 into a parallel beam. The quarter-wavelength plate 10 shifts the phase of the light by λ/4. The light emitted from the first and second light sources 2 and 3 passes through the quarter-wavelength plate 10 twice, i.e., before striking the disk D and after being reflected therefrom. Thus, the phase of the light is shifted by λ/2, with the result that P-polarized light is converted into S-polarized light.
The diffraction grating 7 is a hologram or the like, and varies the focus position of the condenser lens 11 according to the wavelength of the light. The condenser lens 11 focuses the light of the first and second wavelengths on the disk D. In the reflecting and transmitting directions of the BS film 8e, there are disposed light-receiving devices 12 and 13 such as photodiodes, respectively.
In the optical pickup 1 constructed as described above, the P-polarized light of the first wavelength emitted from the first light source 2 is reflected from the dichroic mirror 5 and is thereby directed to the prism 8. The P-polarized light of the second wavelength emitted from the second light source 3 is transmitted through the dichroic mirror 5 and is thereby directed to the prism 8.
The light of the first and second wavelengths passes through the PBS film 8d of the light 8, and is then shaped into a parallel beam by the collimator lens 4. The light then passes through the quarter-wavelength plate 10 and the diffraction grating 7, and is then focused on the recording surface of the disk D by the condenser lens 11. Here, the diffraction grating 7 varies the focus position of the light of the first and second wavelengths according to the type of disk D.
The light of the first and second wavelengths reflected from the disk D passes through the diffraction grating 7, the quarter-wavelength plate 10, and the collimator lens 4, and then enters the prism 8. Now, the light of the first and second wavelengths has passed through the quarter-wavelength plate 10 twice, and thus has been converted into S-polarized light. In the prism 8, the PBS film 8d reflects the S-polarized light, and the BS film 8e reflects part of the light and transmits the remainder thereof.
The light of the first and second wavelengths that has exited from the prism 8 is received by the light-receiving devices 12 and 13, respectively. In this way, from different types of disk D, signals of the corresponding wavelengths can be read by being received by the light-receiving devices 12 and 13.
In the optical pickup constructed as described above, large wavefront aberration in the light that passes through optical thin films, namely the PBS film 8d and the BS film 8e of the prism 8, leads to erroneous recognition of signals. To avoid this, in an optical pickup for use with a CD-ROM or DVD-ROM as the disk D, wavefront aberration is restricted to, for example, within 50 mλ rms.
However, in an optical system for a next-generation DVD, Blu-ray disk, or the like, reading and writing of signals are achieved by the use of blue laser light of wavelengths of 420 nm or shorter (in the 405 nm band). Thus, even wavefront aberration of about 50 mλ rms results in a high incidence of erroneous recognition. Accordingly, the wavefront aberration in the light that passes through optical thin films, namely the PBS film 8d and the BS film 8e, is required to be, for example, within 25 mλ rms.
To achieve this, the last step of the fabrication process of the prism 8 is dedicated to inspection whereby wavefront aberration is measured. In this inspection step, prisms with wavefront aberration larger than 25 mλ rms are rejected as defective, and, inconveniently, this has been keeping the yields of the prism 8 and the optical pickup 1 low.